Barring the Twin Gremlins and the AkZani, Orokin Cells are required to craft all dual secondaries (either as it is or as a Forma). This does not count dual secondaries given away during events.
It seems both the Corpus and the Grineer have already attained a decent amount of Orokin cells, this is due to their continuous excavation of the Orokin ruins which one of them can be seen in the beginning of the game. This resource can then be paid to Tenno as they side accordingly to the opposing factions.
The Orokin Cell is used in constructing most Warframes.
Occasionally, when a boss drops an Orokin Cell it will appear in-game as a mod instead of a yellow resource.